There has been known to use a flow cytometer for classifying and counting cells in blood or particles in urine. This method comprises staining cells in a sample with a staining solution, passing the stained cells one by one through a flow cell under irradiating with excitation light beams and measuring the fluorescence or scattered light generated from the cells to thereby classify and count the cells.
In the analysis with the use of a flow cytometer, it is necessary to calibrate the device so as to obtain precise data. It has been a practice to use fixed cells (for example, red blood cells, bacteria), latex particles or fluorescent particles for the quality control and calibration of a flow cytometer. For example, Japanese Laid-Open Patent Publication No. 6-102152 has disclosed a standard fluid for a flow-type particle image analyzer with the use of an ion exchange resin, while Japanese Laid-Open Patent Publication No. 4-278460 has proposed to use cells, bacteria, latex particles, microcapsules, starch grains, gelatin grains and pollens as marker particles.
However, materials with biological origins such as cells and bacteria show large differences among individuals, which makes it difficult to establish stable qualities. In addition, troublesome procedures (for example, preventive measures against the infection with pathogenic organisms, cold storage) are required in handling these materials.
On the other hand, latex particles cannot be stained in general and, therefore, are not usable in a system of controlling a staining solution. Fluorescent particles bonded to a fluorescent dye are usable in an optical control system but not in a system of controlling staining wherein not only the staining solution per se but also the staining conditions (the amount of the staining solution to be pipetted, temperature, etc.) should be managed. Further, microcapsules, starch grains, gelatin grains, etc. are sometimes different in staining modes from the cells to be assayed. This is seemingly because these particles differ from actual cells in the affinity for the dye contained in the staining solution. It is preferable that a calibration substance shows similar staining behaviors as those of the cells to be assayed, since the calibration and quality control can be facilitated thereby.
The standard fluid disclosed in Japanese Laid-Open Patent Publication No. 6-102152 as cited above is one to be used in a device for photographing and analyzing particles from the image of each particle. That is, this patent discloses no standard fluid to be used in a device for analyzing scattered light intensity or fluorescence intensity.